1. Field of the Invention
The present invention relates to transmitter-receiver units used in Electronic Toll Collection (ETC) systems.
2. Description of the Related Art
In ETC systems for toll roads such as turnpikes, with time-division two-way communication, transmission and reception is performed between an ETC transmitter-receiver terminal station installed at a toll booth and an ETC transmitter-receiver mounted in a vehicle using 5.8-GHz ASK modulated waves. Predetermined tolls can be collected by transmitting and receiving signals.
FIG. 4 shows an example of a transmitter-receiver circuit used in these ETC systems. FIGS. 5A and 5B show the circuit layout of a transmitter-receiver unit with the transmitter-receiver circuit shown in FIG. 4. FIG. 5A illustrates the transmitter-receiver unit viewed from one side. FIG. 5B illustrates the transmitter-receiver unit viewed from the other side.
Referring to FIG. 4, a transmitter circuit 21 includes a local oscillator 22 for generating carrier waves and a modulation unit 23. The local oscillator 22 includes an oscillator 22b controlled by a PLL (phase-locked loop) circuit 22a. The oscillator 22b oscillates at 2.9175 GHz. The oscillator signal becomes a 5.835-GHz carrier wave after passing through a frequency doubler 22c, and the carrier wave is input through a band pass filter 22d to the modulation unit 23. The carrier wave is input through a preamplifier 23a to a modulator 23b and is subjected to ASK-modulation using a modulation signal, thus becoming a transmission signal. The transmission signal is amplified by a power amplifier 23c, and the amplified transmission signal is transferred to an antenna switch 24. Since the antenna switch 24 is controlled so as to connect the power amplifier 23c to a band pass filter 25 when performing transmission, the transmission signal is output through the band pass filter 25 to an antenna 26.
In contrast, the antenna switch 24 is controlled so as to connect the band pass filter 25 to a receiver circuit 27 when performing reception. A reception signal received by the antenna 26 is input from the band pass filter 25 through the antenna switch 24 to the receiver circuit 27. The reception signal is input through a low noise amplifier 27a to a mixer 27b. A 5.835-GHz local oscillation signal, which is generated by the local oscillator 22, is input to the mixer 27b, and hence the reception signal is frequency-converted into a 40-MHz intermediate frequency (IF) signal. The IF signal passes through a band pass filter 27c and an IF amplifier 27d and is ASK-demodulated by a detector 27e. An ASK demodulation signal, which is generated as a result of demodulation, is amplified by a baseband amplifier 27f and output. The output signal is processed by a circuit at a subsequent stage (not shown).
When a transmitter-receiver unit is formed by mounting circuit components of the foregoing transmitter-receiver circuit on a circuit substrate, it is necessary to minimize crosstalk interference since 5.8-GHz signals are used to perform the transmission and reception. Because of this necessity, as shown in FIGS. 5A and 5B, a high frequency circuit including the transmitter circuit 21 (the local oscillator 23 and the modulation circuit 23) and the receiver circuit 27 is formed on one side of a circuit substrate 28, whereas the antenna 26 is formed on the other side of the circuit substrate 28. The antenna 26 is formed by providing an antenna patch 26b on an antenna substrate 26a. The antenna substrate 26a is attached to the circuit substrate 28 with double-sided adhesive tape. A feed pin 29 is soldered at a feeding point of the antenna patch 26b. The feed pin 29 penetrates through the circuit substrate 28 and is soldered at a land at an input side of the band pass filter 25, which is formed at a high frequency circuit side.
In the foregoing known transmitter-receiver unit, the antenna 26 and the high frequency circuit are separately formed on the corresponding sides of the circuit substrate 28. Thus, the radiation characteristics of the antenna 26 become less apt to be influenced by the 5.835-GHz carrier wave used by the high frequency circuit. Because all circuit components of the high frequency circuit are mounted on one side of the circuit substrate 28, dead space in the other side of the circuit substrate 28 is increased. This prevents miniaturization of the overall receiver-transmitter unit. Because the antenna substrate 26a is attached to the other side of the circuit substrate 28 using double-sided adhesive tape and power feeding to the antenna patch 26b is implemented by soldering of the feed pin 29, the components of the antenna 26 increases in number, thus increasing the cost. Due to a difference in thermal expansion coefficient between the antenna substrate 26a and the feed pin 29, the soldered feeding point may crack.